1. Field of the Invention
The present invention relates generally to a communication apparatus and method in a mobile communication system. More particularly, the present invention relates to an apparatus and method for interleaving channels in a mobile communication system.
2. Description of the Related Art
Mobile communication systems have been developed to allow users to enjoy voice service regardless of location. With the rapid progress of communication technologies, the mobile communication systems have now evolved into advanced systems capable of data transmission to meet various users' demands. The advent of the advanced systems capable of data transmission enables transmission of various kinds of data. For example, the advanced systems can support Short Message Service (SMS), Internet service, moving image service, broadcasting service, and so on.
For data service, the mobile communication systems aim at transmitting a larger amount of data at a higher rate. In particular, unicast service such as broadcasting service is required to efficiently use the limited resources, because it must provide various broadcasting contents to a plurality of users. To meet the requirement, many attempts are being made to use Orthogonal Frequency Division Multiplexing (OFDM) rather than the conventional Code Division Multiple Access (CDMA) in providing broadcasting service.
In addition, safe data transmission is most important for the data service. To this end, the mobile communication system transmits data using a particular method, for example, a turbo coding method. Such a data transmission method generally makes use of Hybrid Automatic Repeat reQuest (H-ARQ). In H-ARQ, when a transmitter transmits data, a receiver receives and decodes the data. If the decoding result is bad, that is, if a CRC check result is erroneous, the receiver sends a retransmission request to the transmitter. In response to the retransmission request, the transmitter transmits the transmitted data without modification or modifies the data before transmission. Such a scheme is commonly used in the mobile communication.
However, the foregoing broadcasting service is a real-time unicast service. Therefore, in broadcasting service, the receiver cannot transmit a retransmission request to the transmitter, even though there is an error in the data received from the transmitter. This is because the broadcasting service must allocate different channel resources to a plurality of mobile terminals for data transmission. In other words, the broadcasting service requires more reliable data transmission compared with other services. However, the retransmission of high-speed data causes a decrease in transmission efficiency not only for the broadcasting service but also for other data services.
A detailed description will now be made of a method for transmitting data in a conventional mobile communication system.
FIG. 1 is a conceptual diagram illustrating a method for configuring a coded transmission symbol by coding and interleaving transmission information in a CDMA mobile communication system. With reference to FIG. 1, a description will now be made of a method for configuring a coded transmission symbol by coding and interleaving transmission information in a CDMA mobile communication system.
Transmission information is input to a turbo encoder 100. The turbo encoder 100 encodes the input information at a predetermined coding rate, and uses a coding rate R=1/5 in a CDMA system. In the encoding process, constituent encoders included in the turbo encoder 100 generate parity information pairs using the transmission information, and use it as a redundancy. That is, the turbo encoder 100 outputs coded information U 111, which includes systematic bits that are output without being processed, a first parity symbol pair V0/V0′ 112, and a second parity symbol pair V1/V1′ 113. In other words, the turbo encoder 100 receives one bit and outputs one systematic symbol and four redundancy symbols, satisfying the coding rate R=1/5. The parity symbol pairs 112 and 113 are double in size compared to the coded systematic symbols U 111. The systematic symbols U 111 are input to a first block interleaver 121, the first parity symbol pair 112 is input to a second block interleaver 122, and the second parity symbol pair 113 is input to a third block interleaver 123. Because the symbols input to the interleavers are different in size, the second interleaver 122 and the third interleaver 123 are double in size compared to the first interleaver 121.
The interleavers 121, 122 and 123 interleave their input symbols, and output the interleaved symbols to a serial combiner 130. The serial combiner 130 serially combines the output symbols of the block interleavers 121, 122 and 123, generating symbols 131, 132 and 133.
The symbols generated by serial-combining (or concatenating) the independently interleaved symbols are divided into an initial transmission subpacket (or first transmission subpacket) 141, a primary retransmission subpacket (or second transmission subpacket) 142, and a secondary retransmission subpacket (or third transmission subpacket) 143 according to transmission time slot and slot size, and they are used for initial transmission, primary retransmission and secondary retransmission, respectively. The subpacket transmitted at initial transmission includes the coded interleaved symbols U 131 and a part of the coded interleaved first parity symbol pair V0/V0′ 132. Therefore, when the interleaving is disregarded and only the types of transmission symbols are considered, coded systematic symbols 131 and a part of the first parity symbol pair 132 constituting a redundancy are transmitted during the initial transmission. A part of the first parity symbol pair 132 constituting the redundancy is transmitted during the primary retransmission, and the remaining part of the first parity symbol pair 132 and a part of the second parity symbol pair 133, both constituting the redundancy, are transmitted during the secondary retransmission.
A description will now be made of an interleaving process.
FIGS. 2A through 2C are diagrams illustrating interleaving processes performed in the conventional CDMA system. The interleaving processes performed in the conventional CDMA system will now be described with reference to FIGS. 2A through 2C.
FIG. 2A is a diagram illustrating the input order of data being input to a block interleaver. As illustrated in FIG. 2A, if the total size of input coded symbols to be interleaved is determined, the interleavers determine a size of their input symbols using Equation (1) below to receive the input symbols.Total Size of Coded Symbols=R×2M  (1)
In Equation (1), a horizontal size of the block is denoted by 2M and a vertical size of the block is denoted by R. As shown in Equation (1), the horizontal size is fixed to an exponential power of 2 and a value of R is determined such that the exponential power of 2 should be maximized.
For example, if a size N of the coded systematic symbols U 111 is 3072, the maximum value among the values expressed with an exponential power of 2 is 210. Therefore, a value of the R becomes 3 (R=3). That is, if a size of the input symbols is 3072 in accordance with Equation (1), the first block interleaver 121 has a size of 3×210.
Because the first parity symbol pair 112 and the second parity symbol pair 113 output from the turbo encoder 100 are double in size compared to the coded systematic symbols U 111, a size of the second block interleaver 122 and the third block interleaver 123 is expressed as 3×211, taking Equation (1) into consideration. That is, the size is determined such that R=3 and M=11.
For this size, the input order is determined as shown in FIG. 2A. More specifically, when the block is divided into rows and columns, the number of rows is R and the number of columns is 2M. Therefore, the interleaver inputs coded symbols to a first row 211 among the R rows in a left-to-right direction, and after fully filling the first row 211 with the coded symbols, inputs coded symbols to the next row 212 in the left-to-right direction. This process is repeated until all of the R rows are filled with the coded symbols.
After filling all of the R rows with input symbols in this manner, the block interleaver performs permutation of the columns on a bit reverse order (BRO) basis. This will be described with reference to FIG. 2B. FIG. 2B is a diagram illustrating a process of determining column positions on a BRO basis in a block interleaver.
The BRO interleaving process includes:
a) converting a decimal number indicating the order of a column into an M-digit binary number;
b) BRO-ordering the binary number;
c) re-converting the BRO-ordered binary number into a decimal number; and
d) shifting all symbols in the corresponding column to a column indicated by the decimal number.
A description will now be made of an example of the BRO interleaving process. Assuming that M=11, if a first column is a 3rd column, the order of the first column can be expressed with a binary number ‘00000000011’. If the binary number ‘00000000011’ is subject to BRO ordering, the BRO-ordered binary number becomes ‘11000000000’. If the BRO-ordered binary number ‘11000000000’ is re-converted into a decimal number, the decimal number becomes 1536. As a result, all symbols in the 3rd column are shifted to the 1536th column. On the contrary, the 1536th column is shifted to the 3rd column. This process, as shown in FIG. 2B, is expressed such that BRO(1)=2M/2 and BRO(1)=(2M/4), . . . .
After performing column permutation on a BRO basis, the block interleaver outputs the corresponding symbols. With reference to FIG. 2C, a description will now be made of the output order of the BRO-ordered symbols. FIG. 2C is a diagram illustrating an output (reading) order of symbols in a block interleaver after BRO-based column permutation.
As illustrated in FIG. 2C, symbols are output column by column. That is, although the symbols are written (stored) row by row, the symbols are read (output) column by column. When the columns are designated as 0th column, 1st column, 2nd column, . . . in the left-to-right direction, symbols are output from the top to the bottom of the 0th column. That is, the data is output in the order of columns denoted by reference numerals 220, 221, 222, 223, . . . , 224, 225.
The symbols generated by serial-combining the coded interleaved symbols are modulated according to a predetermined modulation method before being transmitted. The modulation method may include Quadrature Phase Shift Keying (QPSK), 8-ary Phase Shift Keying (8PSK) and 16-ary Quadrature Amplitude Modulation (16QAM). The modulation symbols differ from each other in symbol reliability according to modulation method used for the modulation. Actually, in 8PSK or 16QAM, modulated bits are different from each other in reliability. For example, if 3 interleaved bits ‘b0,b1,b2’ are mapped to one 8PSK symbol before being transmitted, the symbols ‘b0,b1,b2’ are not equal to each other in reliability. That is, the symbol b2 is lower in reliability than the symbols b0 and b1. In addition, if 4 interleaved bits ‘b0,b1,b2,b3’ are mapped to one 16QAM symbol before being transmitted, the symbols b1 and b3 are lower in reliability than the symbols b0 and b2. The reliability is determined depending on a mapping method. Although the low-reliability bit positions can be improved by modifying the mapping method, the modification of the mapping method may cause a decrease in reliability of other symbol positions. That is, there are always some mapped bits whose transmission reliability is lower than that of the other bits.
In this case, if the bits interleaved by the conventional interleaver are mapped to a modulation symbol, adjacent bits might be mapped to the positions having the same reliability. As a result, the bits may be mapped to high-reliability positions in a particular interval, and to low-reliability positions in another interval, deteriorating channel coding performance. Therefore, during high-speed data transmission, a retransmission request may be frequently issued due to the low-reliability symbols. The service that cannot support retransmission, such as broadcasting service, suffers QoS deterioration. In addition, a possible issuance of the retransmission request may result in a service delay and a reduction in channel resource efficiency.
Accordingly, there is a need for an improved apparatus and method for interleaving channels in a mobile communication system.